1. Field of the Invention
This invention relates to a semiconductor device.
2. Description of the Prior Art
Generally, for increasing the blocking voltage (the breakover voltage in the forward direction and the breakdown voltage in the reverse direction) of semiconductor devices such as diodes and thyristors, a very important problem exists in the surface state of semiconductor substrates where a PN junction is exposed. A semiconductor substrate surface is very active and easily influenced by the ambient atmosphere. If moisture and/or ionizable material exist in the atmosphere, they adhere to the substrate surface and make the surface field intensity larger than the field intensity in the semiconductor substrate. Thus, the blocking voltage of a semiconductor device is influenced by the surface state of the semiconductor substrate.
For providing good reproducibility in the blocking voltage of semiconductor devices, it is necessary to arrange that the blocking voltage of a semiconductor device be determined by that of the semiconductor substrate which is simply determined by the impurity concentration. For achieving this, the surface field intensity of a semiconductor substrate should be sufficiently smaller than the field intensity in the substrate. It is known that for decreasing the surface field intensity less than the inside field intensity, the side surface of a semiconductor substrate where a PN junction is exposed can be effectively bevelled with respect to the PN junction. It is also known that in the case of a positive bevel where the higher impurity concentration side region has a larger cross section as it departs from a PN junction, the angle between the PN junction surface and the side edge surface on the lower impurity concentration side is preferably 15.degree. to 60.degree., and that in the case of a negative bevel where the higher impurity concentration side region has a smaller cross section as it departs from a PN junction, the angle between the PN junction surface and the side edge surface on the lower impurity concentration side is preferably 170.degree. to 180.degree. (0.degree. to 10.degree. when seen on the higher impurity concentration side). The details are disclosed in U.S. Pat. Nos. 3,179,860 and 3,361,943 (German Pat. Nos. 1,464,622 and 1,212,215).
In conventional thyristors of a high blocking voltage, both forward and reverse characteristics are needed and a PN junction between a P-type emitter layer and an N-type base layer is formed in a positive bevel of 15.degree. to 60.degree. and another PN junction between the N-type base layer and the P-type base layer is formed in a negative bevel of 0.degree. to 10.degree.. Such structures where a positive and a negative bevel are formed in the side surface of a semiconductor substrate are called double bevel structures since, the bevelled surface is formed in two stages. In the double bevel structure, the area of that principal surface which is on the N-type emitter layer side becomes much smaller than that on the P-type emitter layer side and hence the maximum current capacity is determined by the area of the N-type emitter layer. Thus, semiconductor devices of double bevel structure have a drawback that the maximum current capacity for a semiconductor substrate of a certain dimension is small. Further, in double bevel structure, when the breakover and breakdown voltages are desired to be of equal magnitude the angle for the negative bevel should be about 1.degree. or less. This results in another drawback that high quality techniques are needed for obtaining such bevel angles with good reproducibility.
Thus, for eliminating such drawbacks there has been proposed a method in which the side surface of a semiconductor disk is formed in pulley-shape or V-shape (hereinafter referred as pulley-shape) to form positive bevels for the respective PN junctions, as disclosed in U.S. Pat. No. 3,491,272, especially FIG. 16 thereof. However, few studies have been made on the bevel structure of pulley-shaped side surfaces except to show that the blocking voltage of a semiconductor can be determined by that inside the semiconductor substrate by increasing the depth of the valley portion of the pulley-shape to a certain degree, i.e., decreasing the bevel angle of the PN junction surface and thereby decreasing the intensity of surface electric field and that there would be an appropriate range for the depth of the valley portion since excessive depth invites an increase in the intensity of the surface electric field in the valley portion, reductions in the area of current flow and the mechanical strength. Therefore, it has been difficult to employ the pulley-shaped bevel structure and to design and manufacture semiconductor devices having predetermined blocking voltages for various specifications and to make optimum utilization of semiconductor substrates for a predetermined current flow.